With the advent of magnetic resonance imaging (MRI) techniques, the imaging of soft tissue structures in a non-invasive fashion has become feasible. When a human or animal body is exposed to a strong external time-independent magnetic field (B0), the magnetic moments associated with the spins of the exposed atomic nuclei will become aligned with the direction of B0-field resulting in a total magnetisation to be detected. The direction of this total magnetisation in its equilibrium state is parallel to the direction of the external magnetic field B0. This equilibrium state, however, is not static but, rather, dynamic because the total magnetisation precesses with the so-called Larmor-frequency about the direction of the B0-field.
Upon application of a high frequency (HF) signal having a frequency equal to the Larmor-frequency (resonance frequency) and emerging from a direction different to that of the B0-field, a spin-flip of the nuclei can be observed and associated with the spin-flip, the relaxation time required to relax the spins back to their original alignment with the B0-field can be measured by means of an external coil being tuned in resonance with the HF-signal.
The angle α by which the spins have been deflected by the HF-signal with respect to the B0-field direction is proportional to the time period of the HF-signal and the magnitude of the static magnetic field B0. Subsequent to the spin flip, the total magnetisation precesses about the B0-field with the angle α, and this precessing motion of the total magnetisation may be recorded by the external coil that is oriented perpendicular to the B0-field. The coil outputs a voltage signal whose magnitude is proportional to sin(α), is proportional to the density of the spins and is inversely proportional to the temperature.
If the spins are deflected by an angle α of 90°, a maximum signal response is obtained. Due to the individual spins losing their strict phase-correlation, the recorded signal amplitude decreases exponentially with the relaxation time T2. Simultaneously, the total magnetisation increases exponentially again in the direction of the B0-field towards the equilibrium magnetisation with the relaxation time T1. By means of magnetic gradient-fields switched on at the correct point in time, it is possible to image the two relaxation times in a grey scale encoded image with spatial resolution.
With the discovery of superconductors having a transition temperature above liquid nitrogen temperature, superconducting magnets have become widely used, and thus have rendered MRI-imaging techniques more cost-effective. MRI imaging techniques have so far been predominantly used for imaging soft tissue structures, such as the human brain and other internal organs.
Implants, such as vascular grafts or stents, are predominantly made of biocompatible metals. These metals are still preferred over their polymeric-based competitors. Nickel-titanium alloys are attractive in that they have good fatigue resistance and a memory which brings them to the shape desired upon deployment. Stainless steel or cobalt alloys are other biocompatible materials used for making stents.
There has long been a wish to determine the rate of fluid-flow through the stent lumen as well as the amount of tissue hyperplasia in order to examine the extent of restenosis in each patient during follow-up examinations at intervals after the stent has been implanted. This information would also help stent designers to optimise and improve their stent structures in terms of avoiding restenosis from occurring as well as to help the medical practitioner to exactly determine the extent to which restenosis inside the stent lumen re-occurs after it has been deployed inside the human or animal body in order to specify more precisely those measures for treating the restenosed region in an appropriate and timely manner.
Attempts to MRI-image the blood flow and tissue-ingrowth in the vicinity of a metallic vascular implant are frustrated, or at least impaired, by the ferromagnetic or paramagnetic characteristics of the materials of the implant, which result in artefacts in the images, which reduce the quality of these images down to levels too low to be useful.
On the one hand, these artefacts are thought to be due to differences in susceptibility between metal and tissue resulting in magnetic fields in proximity of the metallic implant being non-uniform and multidirectional, thus destroying the signal response from the HF-pulse in the proximity of the implant. On the other hand, the wavelength of the HF-signals used is such that the implant is, to a certain degree, impenetrable to the HF-signal, i.e. the penetration of the HF-signal through the implant is impaired. Hence, the image of the implant lumen or the body structure therein has been seriously compromised.
These disadvantages reduce the effectiveness of MRI-imaging techniques for imaging patency of vascular metallic implants, and consequently, X-ray fluoroscopy with all its known disadvantages (invasive, ionising radiation) is used instead.
WO-A-96/38083 discloses a probe having at least one pair of elongated electrical conductors, preferably disposed parallel to each other within a dielectric material, and having a pair of ends electrically connected to each other. This probe thus formed is, in a preferred use, introduced into small blood vessels of a patient to facilitate determination of arteriosclerotic plaque using an MRI-imaging technique. This probe, however, is electrically conductive along its entire axial length, thus providing a Faraday screen to minimize dielectric losses between the probe and the surrounding material.
U.S. Pat. No. 6,083,259 addresses the problem of poor visibility of a lumen within a stent. The stents it discloses generally include a series of co-axially aligned circumferential elements and oriented in separate planes spaced axially from each other. Each circumferential element includes a wave-like series of curvatures. Each curvature includes a trough, defined as being that portion of each circumferential element which is most distant from an adjacent circumferential element, and a crest, being defined as that portion of each circumferential element that is closest to an adjacent circumferential element. Each gap between two adjacent circumferential elements is spanned by at least one axial element. The axial elements are either tie bars or double-bend links, such as a S-shaped link. Both the stent and the axial elements are made of the same material. The stent can additionally include enhanced density markers which increase the visibility of portions of the stent when viewed with a medical imaging device, such as a fluoroscope.
U.S. Pat. No. 5,123,917 discloses an intraluminal vascular graft in which separate scaffold members are sandwiched between two PTFE inner and outer tubes. The ring-like scaffold members are made of stainless steel and are expandable upon application of a radially outwardly extending force from the interior of the inner tube. The vascular graft includes no metallic cross-links adjoining two adjacent scaffold members. It is the PTFE inner and outer tubes which hold the vascular graft together.
Another intraluminal graft for placement in a body lumen is disclosed in U.S. Pat. No. 5,122,154. The graft comprises a plurality of stents which may be completely encased in the graft material, the graft material preferably being made of PTFE. In this intraluminal graft, the individual stents are spaced apart axially. The only link between adjacent stents is the PTFE graft material.
EP-A-1 023 609 discloses a stent, said to be compatible with MRI-imaging techniques. The stent has a structural skeleton, which is provided with metal coating portions that function as an inductor and a capacitor. Here, the inductor and capacitor may be co-terminous with the skeleton itself, or may be separate devices attached to the skeleton which are linked in parallel to one another. The inductor and capacitor represent a harmonic oscillator which is tuned in resonance with the HF-signal of a MRI-imaging apparatus.
In case of the skeleton being co-terminous with the inductor and the capacitor, the stent may consist of a structure of two or more layers, in which the first layer is the skeleton, made up of a material having a relatively low electrical conductivity, such as titanium alloys, plastics or carbon fibres, and the coating is a second layer having a very high electrical conductivity in comparison with the first layer and representing the inductor and capacitor material, for example gold or silver. The second, highly conductive layer is cut along circumferential paths during manufacture of the stent. This way, the stent structure comprises several inductors which are connected in parallel. The capacitor is formed at one end of the stent structure by cutting through the highly conductive layer along a relatively short axial path being perpendicular to the cutting paths forming the inductors. In operation, an amplification of the excitation of the nuclei spins by means of the resonance circuit, i.e. the inductor and capacitor, is induced. Therefore, position determination of the stent may be achieved. Furthermore, based on the different excitations inside and outside of the stent, flow rate measurements of the medium flowing through the stent or along the stent can be performed. In the structural skeleton of the stent itself, that is, the first layer, there are no struts in the mesh-structure of the stent which exhibit portions of decreased conductivity, or are entirely severed so that gaps in the mesh-structure would appear. The only gaps are in the second layer for imparting to the stent the property of an harmonic oscillator.
WO-A-01/32102 discloses a tubular structure having a plurality of meander-shaped rings.
In U.S. Pat. No. 5,807,241 a bendable endoscope is disclosed which comprises tube sections so that neighbouring tube sections are completely materially separated from one another via circumferential separating gaps and are only connected to one another by means of a positive fit. By providing an appropriate number of tube sections, a flexible shaft may be formed. The manufacture may be effected by laser-cutting from a rigid tube.
U.S. Pat. No. 5,741,327 discloses a radially expandable surgical stent with radiopaque marker elements in the form of rings attached to the ends of the stent. The radiopaque marker elements include tabs which match the contour of receivers provided at both ends of the stent for secure attachment.
An expandable metallic stent said to be MRI-compatible is disclosed in published US application no. 2002/0188345 A1. The stent has discontinuities of non-conducting material. These eliminate electrically conducting paths in the stent rings. This makes the stent easier to image with MRI. The non-conducting material can include various materials, such as adhesives, polymers, ceramics, composites, nitrides, oxides, silicides and carbides. The discontinuity is preferably shaped that during expansion the discontinuity is placed in primarily a compressive stress. The discontinuities are advantageously placed circumferentially along the stenting rings.